Physiological monitors will often have a testing procedure to check whether the interface between a sensor and the patient being monitored is adequate to acquire a physiological reading. This is especially true with regards to the acquisition of an electrophysiological signal. Typically, an electrophysiological signal is acquired through an electrode which is attached to the patient. The contact between an electrode and a patient's skin can significantly affect the results of an electrophysiological signal. High contact impedance generally causes poor quality recordings due to power interference.
It is a common practice to measure the electrode to skin contact impedance before the start of an electrophysiological recording session. This is done by injecting a small alternating current, Ie, into the electrode and measuring the voltage, Ve, produced across the electrode. The electrode impedance, Ze, may be calculated from the equation Ze=Ve/Ie.
However, most physiological monitors cannot monitor physiological signals during an impedance test because the contact impedance test interferes with the acquisition of the electrophysiological signal. Electrophysiological signals such as EEG, ECG, EOG and EMG are often distorted by the test current utilized during the test. Consequently, the prior art devices have been unable to continuously monitor the contact impedance between the electrode and the patient.
It is not uncommon for electrodes to partially or fully detach from a patient during monitoring, and such an occurrence can seriously distort the electrophysiological signal acquired from the patient. Furthermore, it is often too difficult to visually monitor each electrode on a patient. Consequently, there is a need for an apparatus and a method to continuously monitor the contact impedance between an electrode and a patient without preventing the acquisition of an electrophysiological signal.